It's quite interesting regarding no turbo on this vehicle. I was under the assumption, clearly wrong, that all 1987 300D vehicles were equipped with the turbocharged 603 engine, the same engine as found in my SDL.

I don't have any experience with the naturally aspirated version, but, as others have posted, the air filter is the first place to start. If the engine cannot get sufficient air at full load, it will smoke. Without a turbo, the intake system needs to have no restrictions.

I would highly doubt the IP is the culprit, at this point.